In X-ray computed tomography apparatuses, tube voltage modulation has been demanded in order to reduce the exposure amount. If a tube voltage is simply modulated, the emission properties of a tube current change depending on the tube voltage, and accordingly, the tube current value and the focus size are changed as well.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 2003-163098, for example, discloses that the tube voltage is divided to generate a focus voltage, and the focus size is modulated by the generated focus voltage. Since the focus electrode retains the ground potential, and the tube voltage and the focus voltage have a proportional relationship, the focus size can be stably maintained even if a ripple occurs in the tube voltage.
However, if the tube voltage value significantly changes as occurs in tube voltage modulation, the proportional relationship between the tube voltage and the focus voltage may be deteriorated. Thus, it is difficult to discretionarily control the focus size while performing tube voltage modulation in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-163098.